1. Field of the Invention
The present invention relates generally to a frequency converter for converting a frequency of an input signal by digital signal processing, and in particular, to a frequency converter with an excellent frequency characteristic, capable of converting a sampling frequency.
2. Description of the Related Art
A conventional frequency converter includes a digital down-converter (DDC) and a digital up-converter (DUC). The digital down-converter A/D (Analog-to-Digital)-converts a received RF/IF (Radio Frequency/Intermediate Frequency) signal, and then down-converts the A/D-converted signal to a baseband or demodulation IF signal by digital signal processing. The digital up-converter D/A (Digital-to-Analog)-converts a baseband or modulation IF signal, and then up-converts the D/A-converted signal by digital signal processing in order to generate a transmission RF/IF signal. These frequency converts perform frequency conversion not only on an intact signal but also on a sampling frequency of the signal according to a difference between a sampling frequency needed for RF/IF signal processing and a sampling frequency needed for baseband/modulation/demodulation IF signal processing.
Typically, a multiplier is used as a mixer for frequency conversion of an intact signal, and a decimation filter and an interpolation filter are generally used for suppression of aliasing that occurs while converting a sampling frequency. A general multiplier-based filter requires a plurality of multipliers, causing an increase in circuit size and power consumption. Thus, when a ratio of the sampling frequency to the signal frequency is high, a CIC (Cascade Integrated Comb) filter made by cascading a comb filter and an integrator is generally used.
FIG. 1 illustrates a conventional digital down-converter (DDC) constructed with a CIC filter. Referring to FIG. 1, a digital down-converter 51 includes a quadrature converter (or orthogonal converter) 53, a 1/N-fold decimator 54, a 1/D-fold decimator 55, and a channel filter 56. The quadrature converter 53 is comprised of multipliers 41 for multiplying a sampling signal, obtained by A/D-converting an RF/IF signal S(i) by an A/D converter 52, by signals cos(i) and −sin(i), respectively. The 1/N-fold decimator 54 is comprised of CIC filters 42, and the 1/D-fold decimator 55 is comprised of lowpass filters (FIR (Finite Impulse Response) filters) 43 and 1/D-fold down-samplers 44. The channel filer 56 is comprised of lowpass filters 45. The digital down-converter 51 down-converts the RF/IF signal S(i) to a baseband frequency through the quadrature converter 53, and then down-samples the sampling frequency through the 1/N-fold decimator 54 and the 1/D-fold decimator 55.
FIG. 2 illustrates a detailed structure of the CIC filter for down sampling, illustrated in FIG. 1. The CIC filter includes adders 61, delays 62, subtracters 63, delays 64, and a 1/N-fold down-sampler 65. The adders 61 and the delays 62 constitute a lowpass filter of a section M, while the subtracters 63 and the delays 64 constitute a comb filter of the section M. An input/output signal frequency characteristic of the CIC filter is defined as “H(Z)=(1−Z−MN)/(1−Z−1)”, H(Z) is system function. As represented by a characteristic curve A in FIG. 3, a filter characteristic of a pass band is not smooth. A characteristic curve B is a graph given by expanding a frequency axis of the characteristic curve A.
The CIC filter-based digital down-converter can suppress aliasing without using multipliers, but the pass band characteristic of the filter is not smooth. Therefore, when a frequency bandwidth of an input signal is widened, it is necessary to correct frequency characteristic distortion of the signal, caused by the CIC filter. However, an expansion of the pass band deteriorates a rejection band characteristic of the filter, making it impossible to suppress the aliasing as intended.